Column-row addressable electric microswitch arrays and sensor matrices employing them

ABSTRACT

The present invention relates generally to fabricating two-terminal electric microswitches comprising thin semiconductor films and using these microswitches to construct column-row (x-y) addressable microswitch matrices. These microswitches are two terminal devices through which electric current and electric potential (or their derivatives or integrals) can be switched on and off by the magnitude or the polarity of the external bias. The microswitches are made from semiconducting thin films in a electrode/semiconductor/electrode, thin film configuration. Column-row addressable electric microswitch matrices can be made in large areas, with high pixel density. Such matrices can be integrated with a sensor layer with electronic properties which vary in response to external physical conditions (such as photon radiation, temperature, pressure, magnetic field and so on), thereby forming a variety of detector matrices.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/073,411, filed Feb. 2, 1998, which application isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the fabrication ofelectric microswitches with thin semiconductor films and column-row(x-y) addressable electric switch matrices constructed with suchmicroswitches. These microswitches are two terminal devices throughwhich electric current, electric potential or their derivatives orintegrals can be switched on and off by the magnitude or the polarity ofan external bias. They are made of semiconducting thin films inmetal/semiconductor/metal, thin film configuration. Column-rowaddressable electric micro-switch matrices can be made to cover largeareas, with high pixel density. Such matrices can be integrated with one(or several) additional layer(s) with electronic properties which varyin response to external physical conditions (such as photon radiation,temperature, pressure, x-rays, magnetic field and so on), therebyforming a variety of detector matrices.

[0004] 2. Brief Description of the Prior Art

[0005] Traditional electric switches are electromechanical devices suchas relays for large current, high power applications. On the other hand,there is general interest in high pixel density, column-row addressableelectric microswitches for various sensor applications. Switches madewith discrete mechanical relays are too bulky, too large in size andoften too slow in switching speed for this application. Fewer than 10²channels are typically seen in a single control board in the automationcontrol industry.

[0006] Complementary metal-oxide-semiconductor (CMOS) technology andfield effect transistors have been used to fabricate switching circuitsin large scale integrated circuits (LSICs) on semiconductor wafers.Typicall switching circuits are constructed by a series of field effecttransistors and are known as active matrix arrays. Such microswitcheshave been used in fabrication of high pixel density 2D image sensors andmemory devices. However, the material and the process costs have limitedthe use of such active matrix arrays in large size sensor applications.

[0007] The thin film transistor (TFT) technology on glass or quartzsubstrates, developed originally for the needs of liquid crystaldisplays (LCDs), provide another example of active-mode (AM) microswitchsubstrates. In addition to use in AM-LCDs, a large size, full colorimage sensor made with amorphous silicon (a-Si) p-i-n photocells on a-SiTFT panels was demonstrated recently [J. Yorkston et al., Mat. Res. Soc.Sym. Proc. 116, 258 (1992); R. A. Street, Bulletin of Materials ResearchSociety 11(17), 20 (1992); L. E. Antonuk and R. A. Street, U.S. Pat. No.5,262,649 (1993); R. A. Street, U.S. Pat. No. 5,164,809 (1992)].

[0008] FETs are three-terminal, active devices. Microswitch panels madewith such switch units are often called active matrices. The draincurrent of each FET can be switched on and off by its gate voltage. Theon/off ratio is typically in the range of 10⁴-10⁸.

[0009] As demonstrated in this invention, solid state microswitches canalso be made from two-terminal, passive devices such asmetal/semiconductor Schottky diodes, metal/semiconductor/metal (MSM)devices, p-type semiconductor/n-type semiconductor (p-n) junctiondevices, or p-type semiconductor/insulator or undopedsemiconductor/n-type semiconductor (p-i-n) junction devices. Theelectric current can be switched on and off by the magnitude or thepolarity of an external bias.

[0010] This invention discloses large size, high pixel densitymicroswitch matrices comprising passive devices in MSM structure or instructures of its variations. This invention also discloses a method offabricating such large size, high pixel density microswitch matrices.The semiconductor, which can be either organic or inorganic, is in athin film configuration. Thin film devices made with inorganic materials(such as selenium, germanium, silicon, Ge—Si alloys, ZuS, CdS or CdSe)have been developed for decades, and have been used in manyapplications, including for example photovoltaic energy conversion.Organic diodes in the metal-organic-metal MSM thin film structure havealso been studied [for reviews of MSM devices made with organicmolecules and conjugated polymers, see: James C. W. Chien,Polyacetylene: Chemistry, Physics and Material Science, Chapter 12(Academic, Orlando, 1978); G. A. Chamberlain, Solar Cells 8, 47 (1983);J. Kanicki, in Handbook of Conducting Polymers, T. A. Skotheim, Ed.(Dekker, New York, 1986)]. However, the performance of these earlydevices (as determined by their I-V characteristics) was insufficient toenable use as electric switches.

[0011] With the improvement of both material quality and devicefabrication processes, organic MSM devices with rectification ratios of10⁵-10⁶ were recently demonstrated [D. Braun and A. J. Heeger, Appl.Phys. Letters 58, 1982 (1991); G. Yu, C. Zhang and A. J. Heeger, Appl.Phys. Lett. 64, 1540 (1994)].

[0012] The rectification ratio can be further improved by introducingproper blending processes and by structural variation of the device,such as using a bi-layer semiconducting film or by selecting differentmetals as contacts to improve the carrier injection [I. Parker, J. Appl.Phys. 75, 1656 (1994)]. Such organic MSM devices can be operatedcontinuously over periods in excess of 10⁴ hours at current densities of10 mA/cm² [G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Annual Conferenceof Materials Research Society, San Francisco, April 1995].

[0013] As disclosed in this invention, these thin film MSM devices withhigh recitification ratio can be used for fabricating large area solidstate microswitch boards (panels) with high pixel density.

STATEMENT OF THE INVENTION

[0014] The present invention discloses electric microswitch devicescomprising thin semiconductor films and a methodology for fabricatingthem. The present invention also discloses a methodology for fabricatingcolumn-row (x-y) addressable electric microswitch arrays (matrix panels)with such microswitches as the individual pixel elements. Thesemicroswitches are two terminal devices through which electric current,or electric potential (or their derivatives or integrals) can beswitched on and off by the magnitude or the polarity of an externalbias. They are made of semiconducting thin films inelectrode/semiconductor/electrode, thin film configuration. Column-rowaddressable electric switch matrices can be made in large areas, withhigh pixel density. Such matrices can be integrated with a sensor layerwith electronic properties which vary in response to external physicalconditions (such as photon radiation, high energy particle radiation,temperature, surface pressure, magnetic field and so on), therebyforming a variety of detector matrices.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The detailed description of this invention includes the followingsections:

[0016] Brief description of the Drawings

[0017] Description of the Preferred Embodiments

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] This invention will be further described with reference beingmade to the drawings in which

[0019]FIG. 1 is a cross-sectional view of a solid state microswitch (10)made with a semiconducting film (12) sandwiched between two conductingelectrodes (11, 13) with different or similar work functions;

[0020]FIG. 2A is a cross-sectional view of a switchable two-terminalsensing element (20) comprising a microswitch and a sensing device withelectrical properties (such as conductivity, potential difference) whichvary in response to external environment conditions;

[0021]FIG. 2B is a cross-sectional view of an alternative structure tothat of FIG. 2A in which the sensor layer 24 is arranged closer to thesubstrate (26);

[0022]FIG. 2C is a schematic diagram of the equivalent circuit of FIG.2A and FIG. 2B;

[0023]FIG. 3A shows the structure of a column-row addressable sensormatrix (30) constructed by an array of row electrodes (35) and an arrayof column electrodes (31); at each junction between a column and a rowelectrode is a switchable sensing element similar to that shown in FIG.2;

[0024]FIG. 3B shows the reverse structure to that of FIG. 3A;

[0025]FIG. 4 shows the equivalent circuit of an 8×14 sensor matrix;

[0026]FIGS. 5A and 5B are a set of graphs which show the I-Vcharacteristics (a) and rectification ratio, R_(r) (b) of a microswitchmade in the form of ITO/MEH-PPV/Ca;

[0027]FIGS. 6A and 6B are a set of graphs which show the I-Vcharacteristics (a) and rectification ratio, R_(r) (b) of a microswitchmade in the form of Au/MEH-PPV/Al;

[0028]FIGS. 7A and 7B are a set of graphs which show the I-Vcharacteristics (a) and rectification ratio, R_(r) (b) of a microswitchmade in the form of ITO/MEH-PPV:PCBM/Al;

[0029]FIGS. 8A and 8B are a set of graphs which show the I-Vcharacteristics (a) and rectification ratio, R_(r) (b) of a microswitchmade in the form of Ag/PANI-PAAMPSA/MEH-PPV/Ca;

[0030]FIGS. 9A and 9B are a set of graphs which show the I-Vcharacteristics (a) and rectification ratio, R_(r) (b) of a microswitchmade in the form of ITO/p-i-n/Al in which the semiconductor wasamorphous silicon film;

[0031] FIGS. 10A-10C are a set of graphs which show the characteristicsof dI/dV-V (a), G-V (b), and current integration int IdV-V (c) of amicroswitch made in the form of ITO/MEH-PPV/Ca;

[0032]FIG. 11 shows current vs temperature characteristics of anITO/MEH-PPV/Ca device in the temperature range between −73° C. and +127°C. (200K to 400K);

[0033]FIGS. 12A and 12B show the current image (pixel currentdistribution) on a 10×10 temperature sensor matrix (a); and thecorresponding temperature image from the same matrix (b).

DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] In this description of preferred embodiments and in the claims,reference will be made to several terms which must be defined. One groupof terms concerns the structures of the microswitches and the sensorelements they can control. Cross-sectional views of two embodiments ofthe microswitches are shown in FIG. 1 and FIG. 2. The switch isconstructed using the electrode-semiconductor-electrode thin film deviceconfiguration. Specifically, the switch includes:

[0035] A semiconducting layer (12) comprising organic or inorganicsemiconducting material(s). Examples of organic semiconductors includeconjugated polymers, polymer blends, polymer/molecule polyblends, layerof organic molecules, organometallic molecules, or molecular blends(alloys); or a multilayer structure combining the above materials.Examples of inorganic semiconductors include Si, Se, Ge, Si—Ge alloys,CdS, CdSe, TiO₂, CuO, etc. Examples of organic semiconductors includepolyacetylene (PA) and its derivatives, polythiophene (PT) and itsderivatives, poly(p-phenyl vinylene) (PPV) and its derivatives such asMEH-PPV, fullerene molecules such as C₆₀ and its derivatives, buckytubes, anthracene, tetracene, pentacene, Alq₃ and other metal-chelate(M-L₃) type organometallic molecules and so on. The layer 12 can also bea composite comprising organic and inorganic materials or in the form ofbi-layer or multiple-layers of such materials.

[0036] The layer 12 can be a semiconductor doped with one or moredopants. Extra charges are commonly results from dopants with differentvalance electrons. A doped semiconductor with electrons as majoritycharge carriers is called n-type semiconductor and a doped semiconductorwith holes as majority carries is called p-type semiconductor. Thedoping levels in the layer 12 can distribute inhomogeneously and canchange sign from p-type to n-type (which forms a p-n junction inside 12)or from p-type to undoped region then to n-doped region (which forms ap-i-n junction inside 12).

[0037] Two “contact electrodes” (layers 11, 13) which serve as the anodeand cathode of the switches to extract electrons and holes,respectively, from the semiconductor layer. In some embodiments at leastone of the electrodes (e.g., layer 11 in FIG. 1) is made transparent orsemitransparent. These electrodes are described in more detail below.

[0038] The anode electrode is defined as a conducting material withhigher work function than the cathode material.

[0039] The devices may also include an optional substrate or support 26,as shown in FIG. 2A. This is a solid, rigid or flexible layer designedto provide robustness to the switches and/or to the matrix array ofswitches. In some embodiments the substrate is transparent orsemitransparent at the wavelengths of operation. Glass quartz, polymersheets, or flexible plastic films are commonly used substrates. Wideband semiconductor wafers (such as SiC, SiN) can also be used in someapplications. In these cases, a thin, doped region can also serve as thecontact electrode 21.

[0040] Devices with the “inverted” geometry shown in FIG. 2B are alsouseful In this configuration, light is incident through the electrode 21in contact with the free surface; thus, optically opaque materials canbe used as substrates. For example, by using an inorganic semiconductorwafer (such as silicon) as the substrate 26, and by doping thesemiconductor to “conductive” levels (as defined in the following), thewafer can serve both as the substrate 26 and the contact electrode 25circuitry built directly onto the inorganic semiconductor substrate(using integrated circuit technology).

[0041] Incident light is defined generally to include wavelengths invisible (400-700 nm), wavelengths in the ultraviolet (200-400 nm),wavelengths in vacuum ultraviolet (<200 nm), and wavelengths in the nearinfrared (700-2500 nm).

[0042] Several layers are designated as transparent or semi-transparent.These terms are used to refer to the property of a material whichtransmits a substantial portion of the incident light incident on it.The term “transparent” is often used to describe a substrate withtransmittance over 20% and the term “semitransparent” is often used todescribe a substrate or layer with transmittance between 20% and 1%.

[0043] A “conductive” layer or material has a conductivity typicallylarger than 0.1 S/cm. A semiconducting material has conductivity of from10⁻¹⁴ to 10⁻¹ S/cm.

[0044] A ‘dielectric’ or an ‘insulating’ layer of material has aconductivity typically lower than 10⁻¹⁰ S/cm.

[0045] The “positive” (or “negative”) bias refers to situations where ahigher potential is applied to the anode electrode (cathode electrode).When values of negative voltage are referred to, as in the case of thereverse bias voltages applied to obtain enhanced photosensitivity, therelative values will be stated in terms of absolute values; that is, forexample, a −10 V (reverse) bias is greater than a −5 V (reverse) bias.

[0046] The spectral response of optical image sensing elements which oneoften coupled to its switches is determined by the optical band gap andthe electronic properties (such as carrier mobility) of the sensingmaterial, by the structure of the sensing elements and by thetransmission characteristics of the optical filters, substrates, orother coating layers in the optical path; as demonstrated in theexamples in this application.

[0047] In addition to single band, visible image detection (oftenreferred as black/white, or monochromatic image sensors), there is greatdemand for image sensors with full-color detectivity. Full-colordetection is often achieved by splitting the visible spectrum into threeseparate regions, the red (600-700 nm), green (500-600 nm) and blue(400-500 nm) fundamental colors. A full-color signal can be representedby the intensities of the incident light in these three bands. Afull-color image element thus refers to an image device with threespectral channels in the red, green and blue spectral ranges (sometimes,their complimentary colors, cyan, magenta and yellow, are chosen), andcan provide correct color and light intensity information on theincident light.

[0048] A coating of “black” material (opaque in the spectral range ofinterest) in the area between each sensing pixel can be placed in frontof the photodetector plane, forming a “black matrix”. This coating ishelpful in some situations to further reduce cross-talk between neighborpixels in devices with an unpatterned photoactive organic layer. Blackmatrices have been used in CRT monitors and other flat panel displays toincrease display contrast, and are well known in the display industry.The patterning of the “black matrix” can be achieved with standardphotolithography, stamp, ink-jet or screen printing techniques.

[0049] Electrodes

[0050] In the configuration shown in FIG. 1, a transparent substrate anda transparent electrode are used as one contact electrode.Indium-tin-oxides (“ITO”) can be used as the electrode. Othertransparent electrode materials include aluminum doped zinc oxides(“AZO”), aluminum doped tin-oxides (“ATO”), tin-oxides and the like.These conducting coatings are made of doped metal-oxide compounds whichare transparent from near UV to mid-infrared.

[0051] The electrode can also be made with other doped inorganiccompounds or alloys. These compounds can be doped into metallic (or nearmetallic) form by varying the composition of the elements involved, thevalance of the elements or the morphology of the films. Thesesemiconducting or metallic compounds are known in the art and are welldocumented (e.g., N. F. Mott, Metal-Insulating Transitions, 2nd edition(Taylor & Francis, London, 1990); N. F. Mott and E. A. Davis, ElectronicProcesses in Non-crystalline Materials (Claredon, Oxford, 1979)].Examples of such compounds include the cuprate materials which possesssuperconductivity at low temperatures (so-called high temperaturesuperconductors).

[0052] This electrode can be formed of a conductive polymer such aspolyaniline in the emeraldine salt form prepared using thecounterion-induced processability technology disclosed in U.S. Pat. No.5,232,631 and in Appl. Phys. Lett. 60, 2711 (1992) or other suitabletechniques. The polyaniline film which serves as the electrode can becast from solution with high uniformity at room temperature. The organicconducting electrodes in combination with polymer substrates and organicactive layers allow these photosensors be fabricated in fully flexibleform. Other conductive polymers can be used for the transparent orsemitransparent electrode (11 in FIG. 1 or 21 in FIG. 2A) includepolyethylene dioxythiophene polystyrene sulfonate, (“PEDT/PSS”) [Y. Cao,G. Yu, C. Zhang, R. Menon and A. J. Heeger, Synth. Metals, 87, 171(1997)], poly(pyrrole) or its function derivatives doped withdodecylbenzene sulfonic acid (“DBSA”) or other acid [J. Gao, A. J.Heeger, J. Y. Lee and C. Y. Kim, Synth. Metals 82, 221 (1996)] and thelike.

[0053] A thin semitransparent layer of metals (such as Au, Ag, Al, Inetc.) can also be used as electrodes. Typical thicknesses for thissemitransparent metal electrode are in the range of 50-1000 Å, withoptical transmittance between 80% and 1%. A proper dielectric coating(often in the form of multilayer dielectric stacks) can enhance thetransparency in the spectral range of interest [For examples, see S. M.Sze, Physics of Semiconductor Devices (John Wiley & Sons, New York,1981) Chapter 13].

[0054] A transparent electrode can also be made from metal/conductingpolymer, conducting polymer/metal/conducting polymer or dielectriclayer/metal/conducting polymer structures. The transmission propertiesof these composite electrodes are improved relative to that of a singlemetal layer of the same thickness.

[0055] A metal layer with low optical transmittance can also be used asthe electrode for some applications in which spectral response atcertain wavelengths is of interest. The photosensitivity can be enhancedby fabricating the device in a micro-cavity structure where the twometal electrodes 11 and 13 act also as optical mirrors. Light resonancebetween the two electrodes enhances the photosensitivity at certainwavelengths and results in selective spectral response, similar to thatseen in optical microcavity (optical etalon) devices.

[0056] The “back” electrode 13 in FIG. 1 is typically made of a metal,such as Ca, Sm, Y, Mg, Al, In, Cu, Ag, Au and so on. Metal alloys canalso be used as the electrode materials. These metal electrodes can befabricated by, for example, thermal evaporation, electron beamevaporation, sputtering, chemical vapor deposition, melting process orother technologies. The thickness of the electrode 13 in FIG. 1 (and 11in FIG. 2) is not critical and can be from hundreds of Ångstroms tohundreds of microns or thicker. The thickness can be controlled toachieve a desired surface conductivity.

[0057] When desired, for example, for a photodiode with detectivity onboth front and back side, the transparent and semi-transparent materialsdescribed above can also be used as the “back” electrode.

[0058] As demonstrated in the examples of this invention, microswitcheswith useful I-V characteristics can be fabricated using contact layerswith relatively high resistivity. For example, PANI-PAAMPSA with a bulkresistivity of order 10⁵ Ω-cm can be used as the anode material ofmicroswitches with good switching I-V characteristics. For example, in amicroswitch fabricated in the structure of Mg/MEH-PPV/PANI-PAAMPSA/Ag,the I-V characteristics are defined by the work functions ofPANI-PAAMPSA and Mg. Ag serves only a conductor in the test circuit. Theadvantage of using a contact electrode with high bulk resistivity isthat when the lateral resistance is high enough (by proper selection ofthe bulk resistivity, the thickness of the high resistive anode materialand the dimension of the pixels), the cross-talk between pixels becomessufficiently small that patterning of the high bulk resistivity materialis not necessary.

[0059] Two “contact electrodes” (11, 13) serve as the anode and cathodeof the diodes to inject electrons and holes, respectively, into thesemiconducting layer (12). In the devices with undoped semiconductor aslayer 12, the anode electrode is defined as the one with relativelyhigher work function. In the devices with p-n or p-i-n junction in thelayer 12, the anode electrode is defined as the one in contact with thep-doped region. In all the cases, the anode can be defined as theelectrode with higher potential when the device is in the conductivemode. In addition to traditional metals and alloys, doped semiconductors(both organic and inorganic) can also be used as the contact materialsof 11 and 13. An example of using doped silicon as the anode of organicluminescent device has been given by I. D. Parker and H. Kim, Appl.Phys. Letters 64, 1774 (1994). Examples demonstrating the utility ofconducting polymers such as polyaniline-(camphor sulfonic acid),PANI-CSA, PEDT-PSS, and polypyrrole, PPy, as electrodes inelectroluminescent devices have been disclosed in a number of patentsand publications [Y. Cao et al., U.S. Pat. No. 5,232,631; G. Gustafsson,Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri and A. J. Heeger,Science 357, 477 (1992); Y. Yang, U.S. Pat. No. 5,723,873; G. Heywangand F. Jonas, Adv. Materials 4, 116 (1992); Y. Cao, G. Yu, C. Zhang, R.Menon and A. J. Heeger, Synth. Metal (1997); J. Gao, A. J. Heeger, J. Y.Lee and C. Y. Kim, Synth. Metals 82, 221 (1996)].

[0060] The Semiconductor Layer

[0061] The semiconductor layer is made of a thin sheet of inorganic ororganic semiconducting material. Inorganic materials include Si, Se, Ge,CdS, CdSe, Ta0, Cuo, etc. The semiconductor layer can comprise one ormore semiconducting, conjugated polymers, alone or in combination withnon-conjugated materials, one or more organic molecules, or oligomers.The semiconducting materials also serve as active layers in the sensordevices to which these switches may be completed. The active organiclayer can be a blend of two or more conjugated polymers with similar ordifferent electron affinities and different electronic energy gaps. Theactive layer can be a blend of two or more organic molecules withsimilar or different electron affinities and different electronic energygaps. The active layer can be a blend of conjugated polymers and organicmolecules with similar or different electron affinities and differentenergy gaps. The latter offers specific advantages in that the differentelectron affinities of the components can lead to photoinduced chargetransfer and charge separation; a phenomenon which enhances thephotosensitivity [N. S. Sariciftci and A. J. Heeger, U.S. Pat. No.5,333,183 (Jul. 19, 1994); N. S. Sariciftci and A. J. Heeger, U.S. Pat.No. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci, L. Smilowitz, A. J.Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz, N. S.Sariciftci, R. Wu, C. Gettinger, A. J. Heeger and F. Wudl, Phys. Rev. B47, 13835 (1993); N. S. Sariciftci and A. J. Heeger, Intern. J. Mod.Phys. B 8, 237 (1994)]. The active layer can also be a series ofheterojunctions utilizing layers of organic materials or blends asindicated above.

[0062] The thin films of organic molecules, oligomers and molecularblends can be fabricated with thermal evaporation, chemical vapordeposition (CVD) and so on. Thin films of conjugated polymers,polymer/polymer blends, polymer/oligomer and polymer/molecule blends canoften be fabricated by casting directly from solution in common solventsor using similar fluid phase processing. When polymers or polyblends areused as the active layer, the devices can be fabricated onto flexiblesubstrates yielding unique, mechanically flexible photosensors.

[0063] Examples of typical semiconducting conjugated polymers include,but are not limited to, polyacetylene, (“PA”), and its derivatives;polyisothianaphene and its derivatives; polythiophene, (“PT”), and itsderivatives; polypyrrole, (“PPr”), and its derivatives;poly(2,5-thienylenevinylene), (“PTV”), and its derivatives;poly(p-phenylene), (“PPP”), and its derivatives; polyflourene, (“PF”),and its derivatives; poly(phenylene vinylene), (“PPV”), and itsderivatives; polycarbazole and its derivatives; poly(1,6-heptadiyne);polyisothianaphene and its derivatives; polyquinolene and semiconductingpolyanilines (i.e. leucoemeraldine and/or the emeraldine base form).Representative polyaniline materials are described in U.S. Pat. No.5,196,144 which is incorporated herein by reference. Of these materials,those which exhibit solubility in organic solvents are preferred becauseof their processing advantages.

[0064] Examples of PPV derivatives which are soluble to common organicsolvents include poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene), (“MEH-PPV”) [F. Wudl, P. -M. Allemand, G. Srdanov, Z. Ni andD. McBranch, in Materials for Nonlinear Optics: Chemical Perspectives,edited by S. R. Marder, J. E. Sohn and G. D. Stucky (The AmericanChemical Society, Washington D.C., 1991), p. 683.],poly(2-butyl-5-(2-ethyl-hexyl)-1,4-phenylenevinylene), (“BuEH-PPV”) [M.A. Andersson, G. Yu, A. J. Heeger, Synth. Metals 85, 1275 (1997)],poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene), (“BCHA-PPV”) [seeU.S. patent application Ser. No. 07/800,555, incorporated herein byreference] and the like. Examples of soluble PTs includepoly(3-alkylthiophenes), (“P3AT”), wherein the alkyl side chains containmore than 4 carbons, such as from 5 to 30 carbons.

[0065] Organic image sensors can be fabricated using donor/acceptorpolyblends as the photoactive layer. These polyblends can be blends ofsemiconducting polymer/polymer, or blends of semiconducting polymer withsuitable organic molecules and/or organometallic molecules. Examples forthe donor of the donor/acceptor polyblends include but are not limitedto the conjugated polymers just mentioned, that is PPV, PT, PTV, andpoly(phenylene), and their soluble derivatives. Examples for theacceptors of the donor/acceptor polyblends include but are not limitedto poly(cyanaophenylenevinylene) (“CN-PPV”), fullerene molecules such asC₆₀ and its functional derivatives, and organic molecules andorganometallic molecules used heretofore in the art for photoreceptorsor electron transport layers.

[0066] One can also produce photoactive layers using two semiconductingorganic layers in a donor/acceptor heterojunction (i.e., bilayer)structure or alternation layer structures. In these structures, thedonor layer is typically a conjugated polymer layer and the acceptorlayer is made up of poly(cyanaophenylenevinylene) (“CN-PPV”), fullerenemolecules such as C₆₀ and its functional derivatives (such as PCBM andPCBCR), or organic molecules used heretofore in the art forphotoreceptors and electron transport layers. Examples of thisheterojunction layer structure for a photoactive layer include but arenot limited to, PPV/C₆₀, MEH-PPV/C₆₀, PT/C₆₀, P3AT/C₆₀, PTV/C₆₀ and soon.

[0067] The active layer can also be made of wide band polymers such aspoly-N-vinylcarbazole (“PVK”) doped with dye molecule(s) to enhance thephotosensitivity in visible spectral range. In these cases, the wideband organic serves as both host binder as well as hole (or electron)transport material. Examples include, but are not limited to,PVK/o-chloranil, PVK/rhodamine B and PVK/coronene and the like.

[0068] The photoactive layer can employ organic molecules, oligomers ormolecular blends. In this embodiment, the photosensitive material can befabricated into thin films by chemical vapor deposition, molecularepitaxy or other known film-deposition technologies. Examples ofsuitable materials include but are not limited to anthracene,phthalocyanine, 6-thiophene (“6T”), 6-phenyl (“6P”), Aluminum chelate(Alq3) and other metal-chelate molecules (m-q3), PBD, spiro-PBD,oxadiazole and its derivatives or molecular blends such as 6T/C₆₀,6T/pinacyanol, phthalocyanine/o-chloranil, 6P/Alq3, 6P/PBD and the like.

[0069] Examples of organic molecules, oligomers and molecular blends canbe used for the active layer include anthracene and its derivatives,tetracene and its derivatives, phthalocyanine and its derivatives,pinacyanol and its derivatives, fullerene (“C₆₀”) and its derivatives,thiophene oligomers (such as sixethiophene “6T” and octithiophene “8T”)and their derivatives and the like, phenyl oligomers (such as sixephenyl“6P” or octiphenyl “8P”) and their derivatives and the like, 6T/C₆₀,6P/C₆₀, 6P/Alq3, 6T/pinacyanol, phthalocyanine/o-chloranil,anthracene/C₆₀, anthracene/o-chloranil and the like. For the photoactivelayer containing more than two types of molecules, the organic layer canbe in a blend form, in bilayer form or in multiple alternate layerforms.

[0070] In some embodiments, the active layer 12 comprises one or moreorganic additives (which are optically non-active) to modify and toimprove the device performance. Examples of the additive moleculesinclude anionic surfactants such as ether sulfates with a commonstructure,

R(OCH₂CH₂)_(n)OSO₃ ⁻M⁺

[0071] wherein R represents alkyl alkyllaryl,

[0072] M⁺ represents proton, metal or ammonium counterion,

[0073] n is moles of ethylene oxide typically n=2-40).

[0074] Application of such anionic surfactants as additives forimproving the performance of polymer light-emitting diodes has beendemonstrated by Y. Cao [U.S. patent application, Ser. No. 08/888,316,which is incorporated by reference].

[0075] Other types of additives include solid state electrolytes ororganic salts. Examples include poly(ethylene oxide), lithiumtrifluoromethanesulfonate, or their blends, tetrabutylammoniumdodecylbenzenesulfonate and the like. Application of such electrolyte toluminescent polymers and invention of new type of light-emitting deviceshave been demonstrated in U.S. Pat. Nos. 5,682,043 and 5,677,546.

[0076] In cases where the active layer is made of organic blends withtwo or more phases with different electron affinities and optical energygaps, nanoscale phase separation commonly occurs, and heterojunctionsform at the interfacial area. The phase(s) with higher electron affinityacts as an electron acceptor(s) while the phases with lower electronaffinity (or lower ionization energy serves as an electron donor(s).These organic blends form a class of charge-transfer materials, andenable the photo-initiated charge separation process defined by thefollowing steps [N. S. Sariciftci and A. J. Heeger, Intern. J. Mod.Phys. B 8, 237 (1994)]:

[0077] Step 1: D+A″^(1,3)D*+A, (excitation on D);

[0078] Step 2: ^(1,3)D*+A″^(1,3)(D−A)*, (excitation delocalized on D-Acomplex);

[0079] Step 3: ^(1,3)(D−A)*″^(1,3)(D^(d+)−A^(d−))*, (charge transferinitiated);

[0080] Step 4: ^(1,3)(D^(d+)−A^(d−))*″^(1,3)(D^(+°)−A^(−°)), (ionradical pair formed);

[0081] Step 5: ^(1,3)(D^(+°)−A^(−°))″D^(+°)+A^(−°), (charge separation)

[0082] where (D) denotes the organic donor and (A) denotes the organicacceptor; 1,3 denote singlet or triplet excited states, respectively.

[0083] Typical thickness of the active layer range from a few hundredAngstrom units to a few thousand Angstrom units; i.e., 100-5000 Å (1Angstrom unit=10⁻⁸ cm). Although the active film thicknesses are notcritical, device performance can typically be improved by using thinnerfilms with optical densities of less than two in the spectral region ofinterest.

[0084] The microswitch (10) may also include an optional substrate orsupport 14, as shown in FIG. 1. This is a solid, rigid or flexible layerdesigned to provide robustness to the diodes and/or to a matrix array ofdiodes. Glass, quartz, polymer sheets or flexible plastic films aresubstrates commonly used. Semiconductor wafers (such as Si, GaAs, SiC,SiN) can also be used as the substrate 14. In these cases, a thin, dopedregion of the substrate can also serve as the contact electrode 11.

[0085] The current-voltage (I-V) characteristics of the devices shown inFIG. 1 are typically asymmetric. As demonstrated in the Examples, therectification ratio (defined as the ratio of forward current to reversecurrent at a given bias magnitude) can be as high as 10⁶-10⁷; i.e.,conductive in the forward bias direction and insulating at zero and inreverse bias. Devices with two terminals, such as that of FIG. 1, andwith strong asymmetry in I-V characteristics are traditionally calleddiodes (passive devices) and are represented by a symbol →, in which thearrow denotes the direction of current flow. The switching property canbe characterized by the rectification ratio I(V)/I(−V), or the switchingratio (a current ratio at two given voltages I(V₁)/I(V₂) in whichI(V₁)>I(V₂)). A special situation is V₂=0. As demonstrated in theExamples of this invention, the switching ratio I(V₁)/I(V₂) can behigher than 10¹⁰ for V₂ close to 0V.

[0086] The MSM device (10) shown in FIG. 1 can be used as an electricswitch to switch on and off a sensor device in series with the switch,forming a voltage switchable sensor unit. Two geometric structures ofthe sensor unit 20 are shown in FIGS. 2A and 2B. This unit 20 includessubstrate 26, a sensor made up of electrodes 25 and 23 and sensingelement 24. The switching function is provided by electrodes 21 and 23and semiconductor layer 22. FIG. 2C shows their equivalent circuit.Since the conductivity or electric potential (or time derivatives orintegrals) of the sensor layer are designed to change in response toexternal physical conditions (for example, temperature or magneticfield), this sensing circuit can be used as a sensing element which canbe switched on in forward bias and be switched off at zero and reversebias.

[0087] Many physical effects can be detected by selecting the sensinglayer. For example, the resistivity of the sensing layer can changedirectly in response to changes in the temperature, magnetic field(magnetoresistivity or Hall effect), incident light intensity, incidentmicrowave radiation strength or in response to X-ray or other highenergy fluxes (photoconductive effect) and so on. Alternatively,external environmental changes results in a built-in electric potentialin the sensing layer, which in turn leads to the forward current changein the circuit unit shown in FIG. 2C. Examples include the piezoelectriceffect (voltage change in response to pressure change), thethermoelectric effect (voltage change in response to temperaturechange), and the photovoltaic effect (voltage change in response tochange in incident light intensity), etc. The sensing element (24) shownin FIG. 2 can also be used for another type of sensing applications, inwhich the derivative and integral of the current vary in response tochanges in the external environment. As demonstrated in Example 7, theMSM microswitches shown in FIG. 1 also exhibit switching characteristicson dI/dV, the integral of I(V), and time derivative and integral of thedevice current.

[0088] When the sensor (20) is used for light wave or radio wavedetection, one of the electrodes (25, 21) should be transparent orsemitransparent to the incident electromagnetic wave. When the wave isincident from the substrate side, the substrate needs to be transparentor semitransparent to the incident wave as well.

[0089] Column-row addressable, two-dimensional (2D), passive sensormatrices can be constructed with the voltage-switchable sensor elements,as shown in FIG. 2. Two matrix structures are shown in FIG. 3. Theirequivalent circuits (in 8 row, 14 column form) are shown in FIG. 4. Inthese sensor matrices, the electrodes (31, 35) are typically patternedinto rows and columns perpendicular to each other. In the case of FIG.3A, an array of the column electrodes (31) is first deposited andpatterned on the substrate (36). Then, a sensor layer (32) is depositedor cast. When the lateral conduction of the sensor layer between thepixels is sufficiently low, this layer does not have to be patterned.Then a metal layer or a doped semiconductor layer (33) is deposited (andpatterned, if necessary); (33) serves as the second electrode for eachof the sensors, and it serves as the contact electrode to the switchingdiodes. As specifically noted, conducting polymers and other dopedsemiconductors with proper work function can be used for layer 33. Whenthe lateral resistance of the material comprising layer 33 is highenough, the cross-talk between neighboring pixels is negligible. In suchcases, 33 need not be patterned, a simplification which leads toconsiderable advantage in manufacturing cost and reliability. Theswitching layer (34) is then deposited. Finally, the array of the columnelectrodes (35) is fabricated on top of the sensing layer to completethe column-row addressable sensor array.

[0090] Patterning of the semiconductor layers (32, 34) is not necessarywhen the lateral resistance of the material comprising that layer ishigh enough. Thus, continuous sheets can be used for the sensor matrix.Each cross-point of the row and column electrodes defines a sensingelement (pixel) with device structure similar to that shown in FIG. 1 orFIG. 2. The row and the column electrodes also serve as the contactelectrodes for the microswitches and the sensing devices. The electrode33 can be either a single-layer or a bi-layer, to meet the contact needsfor both switching layer 32 and sensing layer 34. When highly conductivemetals are used for 33, this layer needs to be patterned into isolatedpixels as that shown in FIG. 3. However, a thin layer of metal can beprepared as the electrode 33 in the form of granular particles with adensity below percolation threshold, the resistance in lateral directionis large enough that no patterning of the electrode 33 is necessary. Asshown in Example 11, such a thin, discontinuous metal film stillprovides the work function needed for the switching devices.

[0091] When doped semiconductor layers with relatively high lateralresistivity are used for 33, patterning of 33 can be avoided for certainapplications, and a thin continuous coating can be used. The active areaof each pixel is defied either by the widths of the row and columnelectrodes (31, 35), or defined by the patterned size of electrode 33,whichever is smaller.

[0092] The device structure can be reversed so that the switching layeris closer to the substrate, as shown in FIG. 3B. The preference isdependent on the process simplicity or the type of sensor fabricated.For example, for a pressure sensor which senses the mechanical pressurenear the device pixels, FIG. 3B is the better choice since the sensorlayer is closer to the free surface. For an IR sensor in which aninorganic semiconductor layer (e.g., Ge) is used for IR sensing, thestructure of FIG. 3A is favored, since the metal layer (33) can bepatterned with conventional photolithography technology on top of the IRsensor (Ge) layer.

[0093] The common electrode 33 provides the work function necessary forthe switching diodes. Since high resistivity polymers (such as PANI) anddoped inorganic semiconductors can provide this function, such materialscan be used as the common electrode between the sensing layer and theswitching layer. In some applications, patterning of the commonelectrode layer can also be eliminated.

[0094] These microswitch-based passive sensor matrices can be operatedand addressed using schemes similar to those developed for lightemitting diode matrices. One practical driving scheme is to apply apositive bias across the row and column electrode selected (for example,between row electrode 2 and column electrode B in FIG. 4), leaving therest floating. This is often achieved with analog multiplexers connectedto the row and column electrodes. The pixel at the cross-point of thetwo electrode is then switched on with an applied forward bias largerthan the turn-on voltage. Since there is always a diode in reverse biasin the parallel paths seen from 2 to B (except the path at thecross-point), the leakage currents from the parallel paths arenegligible. Thus, pixel 2B is selected. The current or its integral(charge) or derivative tested in the external test loop connecting 2B isthus sensitive to the physical condition near the pixel 2B. The image ofthe entire sensor can be electronically recorded by selecting each pixelof the array in time sequence.

[0095] Another driving scheme is achieved by connecting the row and thecolumn electrodes to digital gates which provide only two possiblevoltage states, “high” and “low”. This is often accomplished via digitaldecoder circuits or digital shift registers. In this driving scheme,three biasing conditions at V+, 0V and V− exist for each sensing pixel.For example, if at an instant of time, row 2 is selected low and columnB is selected high, then pixel 2B is under forward bias and is selected.The remainder of the pixels in the row 2 can be actively turned off byapplying the same potential as that of 2 to all the columns except B.These pixels are thus under zero bias. The remaining pixels in thecolumn B can also be actively turned off by applying the same potentialas that of column B to all rows except 2. In this way, all the pixelsexcept 2B will be subject to either zero bias or reverse bias, andtherefore will not contribute to the current in the external circuit.For the pixels under zero bias, an even larger switching ratio,I(V_(on))/I(0V) is achieved than that given by the rectification ratioI(V_(on))/I(−V_(on)).

[0096] Another driving scheme is achieved by fixing all the row(orcolumn) electrodes at a given potential (such as at 0V) for read out(such as connected to current-voltage converters or currentintegraters). The colunm(or row) electrodes are scanned one-by-onebetween the same potential applied to the row(column) electrodes and thepotential corresponding to V_(on). This operation can be realized by adigital shift register or with a digital decoder. In this drivingscheme, a column(row) is switched on at a given time, leaving the restcolumns(rows) under zero bias. The microswitches connected to the offcolumns(rows)are thus in the off state. As shown in the Examples, thecurrent switching ratio I(V_(on))/I(0V) can be over 10 ⁹ times, betterthan the rectification ratio I(V_(on))/I(−V_(on)) Since the data areread out column(row) by column(row), the time need for read out theentire matrix can be much faster than that with the point scan schemes.

[0097] There are two common approaches to column and row selections. Oneis the so-called shift-registration, in which the pixels are scannedline-by-line in time sequence. This addressing method has beenfrequently used in emissive displays made with passive light emittingdiodes and in CCD cameras. The other addressing scheme is the so-called“random addressing”, in which the row and column electrodes are encodedand are selectable with a binary decoding circuit. This addressingscheme has frequently been used in memory chips in the computerindustry. This scan scheme is interested by its selectability of sensingarea with faster frame time. Both addressing schemes can be used toaddress the microswitch boards and the integrated sensor matrices shownin FIGS. 3 and 4.

[0098] The switch matrices disclosed in this invention can be used notonly for sensor applications (picking up signals from each pixellocation), but also for delivering electric signals (current, charge,voltage and their variations) to every pixel locations. Corporating suchmicroswitches with a dielectric layer with electric memory property,readable and writable momory devices can be constructed. Corporatingsuch a switch with a thin film with eletro-optical properties, devicescan be made for electric-optical transformation.

EXAMPLES

[0099] This invention will be further described with the reference tothe accompanying examples. These examples are presented to illustrateways of practicing the invention but are not to be construed aslimitations to the claims.

Example 1

[0100] MSM devices were fabricated in the structure shown in FIG. 1. Theanode electrode (11) used in this Example was an indium doped tin oxide(ITO) layer with work function close to 4.8 eV and surface resisitivityof ˜20 Ω/square. A 7 mil Mylar film was used as the substrate (14) forthis device. A thin layer of MEH-PPV (˜1500 Å) was spin cast onto theITO electrode at room temperature. Details on the synthesis and processof MEH-PPV can be found in literature [F. Wudl, P. M. Allemand, G.Srdanov, Z. Ni, and D. McBranch, in Materials for Nonlinear Optics:Chemical Perspectives, Ed. S. R. Marder, J. E. Sohn and G. D. Stucky(American Chemical Society, Washington, D.C., 1991) p. 683]. A thinlayer of Ca (500-5000 Å) was then thermally evaporated as the cathodeelectrode (the Ca was typically over-covered by a protective layer Al).The active area of each device was 0.1 cm².

[0101]FIG. 5A shows the device current as a function of bias voltage.Forward bias is defined as positive voltage applied to the ITO contact.The I-V characteristics in forward bias can be classified into threeregions. A very small current was detected below 1.3 V (e.g. ˜0.4 nA/cm²at 1V). In the range of 1.3-2 V, the forward current increasedexponentially with the biasing voltage, by approximately five orders ofmagnitude. For V>2 V, the rate of increase of the forward currentdiminished; the forward current in the high bias region was dominated byboth tunneling and space charge limited transport [D. Braun and A. J.Heeger, Appl. Phys. Lett. 58, 1982 (1991); I. D. Parker, J. Appl. Phys75, 1656 (1994)].

[0102] In reverse bias, the current remained nearly constant at thelevel of 10⁻¹¹ A/cm² for several volts. The rectification ratio (R_(r))of this device is plotted in FIG. 5B. R_(r) ˜4×10⁶at 3V.

[0103] MSM devices were also fabricated with P3HT, Alq3, PPV, and C₆₀ asthe semiconducting layer. Similar device performances were observed.

[0104] MSM devices similar to that shown in FIG. 1 were also fabricatedin 4.0 cm×6.4 cm size. Similar device performance was observed.

[0105] This example demonstrates that electric microswitches can beconstructed in the thin film MSM configuration. These two-terminal,passive devices can be switched to the conductive state (“ON” state) byapplying a forward bias (with higher potential on the anode electrode),and switched to the non-conducting state (“OFF” state) by applying areverse bias or a zero bias. Rectification ratios higher than 10⁶ weredemonstrated at 3 V. The ON/OFF switching ratio under I(5V)/I(0V) iseven higher, approaching approximately 10⁹ (see, FIG. 5A).

[0106] This Example also demonstrates that organic semiconductors can beused to fabricate microswitches at room temperature, using low costprocesses (such as casting, ink-jet printing, screen printing, orthermal evaporation) applicable to fabrication of arrays over largeareas.

[0107] Moreover, the microswitches demonstrated in this Example werefabricated on flexible mylar substrates and in flexible form. Thismechanical flexibility is unique compared to commercialized activeswitching panels made with TFTs (the high temperature fabricationprocess processes prohibit the use of flexible substrates).

Example 2

[0108] The experiments of Example 1 were repeated with Au as the anodeand Al as the cathode. The device size was reduced to ˜0.0004 cm²,comparable to that of a pixel in a microswitch matrix for image sensorapplications. The I-V characteristics and the rectification ratio areshown in FIGS. 6A and 6B. The rectification ratio and the ON/Off ratioat 5V and 0V were ˜5×10⁵ and >10⁷, respectively.

[0109] Similar devices were fabricated with different metal cathode,including Sm, Y, Pr, MgAg, MgAl, Li, Ba, Ag, Cu, In, Hf etc. Similarswitching effect was observed in each case.

[0110] Similar devices were fabricated with a thin buffer insertingbetween the semiconducting layer and the cathode layer. Examples of thebuffer layer include inorganic compounds such as LiF, BaO, BaF₂, Li₂O.Organic molecules can also be used for the buffer. Examples include OCAand its derivatives. Similar switching effect was observed in each case.

[0111] Organic molecules were also used for the buffer. Examples includeanionic surfactants such as ether sulfates with a common structure,

R(OCH₂CH₂)_(n)OSO₃ ⁻M⁺

[0112] wherein R represents alkyl alkyllaryl,

[0113] M⁺ represents proton, metal or ammonium counterion,

[0114] n is moles of ethylene oxide typically n=2-40).

[0115] When a buffer layer comprising these organic molecules was placedbetween the Al cathode and the semiconducting MEH-PPV, improved deviceperformance was observed.

[0116] Similar devices were fabricated with different metal anodes,including Au, Cr, Ag, Pt etc. Conducting polymers (such as PANI-CSA,PEDOT-PSS) were also used as the anode material. In each case, similarswitching performance was observed. In the microswitches fabricated inthe Au/MEH-PPV/Ca/Al configuration, a forward current of 200 A/cm² wasachieved under pulsed operation. Comparing this value to the devicecurrent of 10⁻⁹ mA/cm² at ˜zero bias, a switching ratio of >10¹¹ wasdemonstrated.

[0117] This example demonstrates that metals with work functionscovering a broad range can be selected as the anode and cathodematerials. This example also demonstrates that stable metals (such asAu, Ag, Al) and conducting polymers can be used as the electrodematerials for the microswitches. This example also demonstrates that abuffer layer can be inserted between the semiconductor layer and thecathode layer.

Example 3

[0118] The experiments of Example 1 were repeated with ITO as the anodeand Al as the cathode. An MEH-PPV:PCBM blend film (1:1 weight ratio) wasused as the semiconducting layer 12, which was spin cast from solutionin xylenes (concentration of approximately 0.5 wt %). PCBM is afullerene molecule with molecular structure and chemical propertiessimilar to C₆₀ (a form of C molecule in buckyball shape). Details aboutits synthesis and characterization has been documented in literature [J.C. Hummelen, B. W. Knight, F. Lepec, and F. Wudl, J. Org. Chem. 60, 532(1995)]. The I-V characteristics and the rectification ratio are plottedin FIGS. 7A and 7B. The rectification ratio was larger than 4×10⁵ forbias over 1.5 V and the ON/Off ratio operating at 2V and 0V was >3×10⁷.

[0119] Similar results were obtained from ITO/MEH-PPV/C₆₀/Al,ITO/TPB/Alq3/Al devices in which the semiconducting layer was in abi-layer structure reminiscent of the so-called heterojunction structurein inorganic devices.

[0120] This Example demonstrates that the semiconducting layer 12 in themicroswitches can be a blend, a composite, a bi-layer or a multilayerstructure. This Example also demonstrates that, due to the chargetransfer effect between MEH-PPV and PCBM, the forward current turn-onvoltage was significantly reduced, thereby achieving a highrectification ratio at relatively low bias.

Example 4

[0121] The experiments of Example 1 were repeated with Ag as the anodeand Ca as the cathode. The device size was ˜0.0014 cm², reminiscent of a0.35 mm×0.4 mm pixel of a microswitch matrix. A PANI-PAAMPSA layer (300Å) was inserted in between the Ag and the MEH-PPV layer as a bufferlayer. The I-V characteristics and the rectification ratio are plottedin FIGS. 8A and 8B. The rectification ratio was higher than 10⁶ at 6V.The ON/Off ratio at 6V and 0V was greater than 2×10⁸. The PANI layeralso reduced the incidence of electrical shorts, thereby improvingfabrication yield and device operating lifetime considerably.

[0122] This experiment was repeated with the thickness of the bufferlayer varied between 100 and 3000 Å. Similar device performance wasobserved.

[0123] This experiment was repeated with a buffer layer made with PANIwith different resistivity (from 10² S/cm to 10⁻⁶ S/cm). The resistivitywas varied either by choosing different counterions (such as CSA,PAAMPSA) or by blending the conductive PANI with an electrically-inerthost polymer (such as PMMA). Similar device performance was observed.

[0124] This example demonstrated that a buffer layer can be inserted atthe S-M interface to improve the device performance (such as reductionof device shorts, enhancement of device stability). The buffer layer canalso be used to modify the M-S interface and thereby improve the deviceI-V characteristics. The current turn-on in this Example was determinedby the work function of the PANI (˜5 eV), rather than by the Ag (˜4.3eV) metal layer. This example also demonstrates that the resistivity ofthe buffer layer can be varied in a broad range.

Example 5

[0125] Devices were fabricated in the form of ITO/PANI-PAAMPSA/Au.Linear I-V characteristics were observed in both biasing directions.Ohmic contacts at both ITO/PANI-PAAMPSA and metal/PANI-PAAMPSAinterfaces were explored.

[0126] This example along with the Example 4 demonstrate that a highresistive PANI layer can be used as the common electrode 23 and 33 (seeFIG. 2 and FIG. 3) to serve both as the anode for the microswitch andthe ohmic contact to the sensors 24 and 34.

Example 6

[0127] The experiments of Example 1 were repeated with ITO as the anodeand Al as the cathode. An amorphous Si film was used as thesemiconducting layer. The amorphous Si film was prepared by chemicalvapor deposition, and was in p-type/undoped/n-type three layerconfiguration. The doping was achieved by ion implantation. The I-Vcharacteristics and the rectification ratio are plotted in FIGS. 9A and9B. The rectification ratio was ˜3×10⁵ at 2 V and the ON/Off ratio at 2Vand ˜0V was >5×10⁶.

[0128] This example demonstrates that inorganic semiconductor films canalso be used as the semiconducting layer 12, 22, 32 in the microswitchesof this invention.

Example 7

[0129] Microswitches were fabricated similar to that shown in Example 1.The device differential current dI/dV was measured as a function ofbiasing voltage; the data are shown in FIG. 10A. The device conductanceG (=I/V) as a function of bias voltage was measured; the data are shownin FIG. 10B. The integrated current, Int_IdV, as a function of biasingvoltage was measured and shown in FIG. 10C. Switching behavior similarto that of I-V (see FIG. 5A) were observed for all these quantities.

[0130] The time derivative and integral of the device current I werealso tested, switching behavior identical to the device current I (FIG.5A) were observed.

[0131] This example demonstrates that the microswitches 10 can not onlybe used for switching device current I, but also for switching otherphysical parameters related with the current (for example, the variousderivatives and the integrals of the current).

Example 8

[0132] ITO/MEH-PPV/Ca microswitches were fabricated in the structureshown in FIG. 1. The thickness of the MEH-PPV was ˜2000 Å. The devicecurrent at 3V was measured as a function of ambient temperature; thedata are plotted in FIG. 11, which shows that the current follows therelation, exp(−T₀/T), with T₀˜0.34 eV.

[0133] Microswitches in the same configuration were also fabricated in10×10 column-row addressable matrix form similar to that shown in FIG.3, but without layer 33 and 34. The pixel area was 0.35 mm×0.4 mm andthe pitch size was 0.625×0.625 mm². The matrix was scanned with electricpulses between 0 and 3.0 V following the first driving scheme describedin the Description of the Preferred Embodiments. A point heat source wasplaced on top of the pixel (5,5), which results in a temperaturedistribution on top of the 10×10 temperature sensor. The device currentdistribution and its corresponding surface temperature distribution areshown in FIG. 12.

[0134] This Example demonstrates that the microswitches 10 made withsemiconducting films with proper thickness can be used as a temperaturesensor. The forward current of the microswitches varies withtemperature. Column-row addressable matrices fabricated with suchmicroswitches can be used as a temperature sensor to image thetemperature distribution over the device surface.

Example 9

[0135] A voltage switchable infrared sensor element was fabricated inthe structure shown schematically in FIG. 2A. The device was fabricatedon a glass substrate with films in the following order:ITO/MEH-PPV/Ca/Au/InSb/Au. Semitransparent Au (200 Å) was used as theelectrode 25, which allowed infrared IR radiation to pass through andreach the IR sensor layer. A bi-layer metal (Ca/Au) was used as thecommon electrode 23.

[0136] In addition to its good rectifying I-V characteristics, theITO/MEH-PPV/Ca device is also a good light emitting device withred-orange color. Since its emission intensity is proportional to theforward current passing through the device over a broad current range,the integrated device in this example serves as an infrared indicator.The output orange light intensity reflects the intensity of the infraredradiation.

[0137] This example demonstrates that voltage switchable IR sensors canbe constructed by integrating a microswitch and a thin film IR sensor.When the integrated sensor is biased in forward direction, the device isswitched-on with high photosensitivity. The same device at zero andreverse bias does not respond to the IR radiation. This switchingcharacteristic makes the device an ideal element in the construction ofx-y addressable, passive IR sensor matrices. When a visible lightemitting diode is used as the microswitch, the integrated device (thematrix) functions as an IR indicator.

[0138] This example also demonstrates that by replacing the sensor layerwith other materials, sensors and sensor matrices sensitive to radiationin other wavelengths of the electromagnetic spectrum (such as UV, X ray,far-infrared, microwave and radiowave) can be constructed. Sensorssensitive to magnetic field, to mechanical pressure and to acousticwaves can also be fabricated using this general principle.

Example 10

[0139] A voltage switchable X-ray sensor element was fabricated in thestructure shown schematically in FIG. 2A. The device was fabricated on aglass substrate with films in the following order: ITO/MEH-PPV/Al/Se/Al.Selinium was used as the X-ray sensing layer, its electriccharacteristics under X-ray radiation has been disclosed in PhysicsToday, November 1997. A thin Al layer (500 Å) was used as the electrode25, which is transparent to X-ray radiation. Aluminum (2 μm) was used asthe common electrode 23.

[0140] The X-ray radiation results in the current increase (following alinear relation) in the sensing layer 24 made with Selenium. The pixelresistance of the Al/Se/Al device is thus falls to the range of 10³-10⁸Ω/cm² (dependent on radiation intensity) which is much larger than theforward resistance typically <1 KΩ/cm² of the microswitch constructed byITO/MEH-PPV/Al, but much smaller than the switch resistance at zero orreverse bias. The forward current of the integrated X-ray sensor element(ITO/MEH-PPV/Al/Se/Al) is thus reflecting the X-ray intensity, and theswitching characteristics of the integrated device allows it to be usedin fabrication of X-Yaddressable X-ray sensing matrices as described inthis invention.

[0141] This example demonstrates that voltage switchable X-ray sensorscan be constructed by integrating a microswitch and a thin film X-raysensor. When the integrated sensor element (pixel) is biased in forwarddirection, the pixel is switched-on, and the current strength isproportional to the intensity of the X-ray radiation. The same device atzero and reverse bias does not respond to the X-ray radiation. Thisswitching characteristic makes such integrated devices ideal pixelelements for constructing x-y addressable, passive X-ray sensormatrices. When a visible light emitting diode is used as the microswitch(for instance, replacing the Al electrode 23 with Ca/Al bi-layerelectrode), the integrated device shown in this example or a matrix ofsuch integrated devices), the X-ray intensity can be reflected by thevisible electroluminescent intensity of the microswitch, forming a X-rayindicator.

[0142] This example along with Example 9 also demonstrate that byplacing a sensor layer in which its electric conductivity or itsvariations varies with environmental conditions, a variety of sensordevices (e.g., radiation sensors sensitive to a certain range ofelectromagnetic spectrum such as UV, X ray, far-infrared, microwave andradiowave) can be constructed. Sensors sensitive to magnetic field, tomechanical pressure and to acoustic waves can also be fabricated usingthis general principle.

Example 11

[0143] Microswitches were fabricated in the form of Au/MEH-PPV/Ca/Al onglass substrates. The thickness of the Ca layer is varied between 1 nmto 1000 nm. Lateral resistance (resistance in parallel to the substratesurface) measurement of the bare Ca film on a glass substrate revealedthe Ca film forms discontinuous particle for thickness below approximate10 nm (the detail threshold is related to the surface roughness of thesubstrate). The I-V performance of the microswitches with Ca thicknessless than 10 nm is similar to that with Ca film of thickness larger than10 nm, which is determined by the work function of the Ca, rather thanthat of Al.

[0144] This example suggests that a thin metal layer inlaterally-discontinuous granular form can be used as the electrodematerial to define the I-V performance of the microswitch. Such metallayers provide novel thin films of high conductivity in verticaldirection and insulating in lateral direction. They are useful infabrication contact layer 23, 33 (the common electrode of the sensor andthe microswitch) without patterning.

What is claimed is:
 1. A microswitch array comprising a plurality ofelectric microswitches, each member of the plurality of microswitchesbeing in first electrode/semiconductor/second electrode layer form withthe semiconductor layer being a unit body shared by the members of thearray.
 2. The microswitch array of claim 1 wherein the plurality ofmicroswitches are configured in an x-y-addressable array.
 3. Themicroswitch array of claim 1 wherein the semiconductor is an inorganicsemiconductor.
 4. The microswitch array of claim 1 wherein thesemiconductor is an organic semiconductor.
 5. The microswitch array ofclaim 2 wherein the semiconductor is an organic semiconductor.
 6. Themicroswitch array of claim 4 wherein the organic semiconductor isselected from the group consisting of conjugated organic semiconductingpolymers, conjugated organic semiconducting polymer blends,semiconducting organic molecules, semiconducting organometallicmolecules, molecular blends or semiconducting organic molecules andmultilayer structures of such materials.
 7. The microswitch array ofclaim 1 wherein at least one of the electrodes is a metallic electrode.8. The microswitch array of claim 1 wherein at least one of theelectrodes comprises a conductive organic polymer.
 9. The microswitcharray of claim 1 wherein at least one of the electrodes comprises abuffer layer adjacent to the semiconductor layer.
 10. The microswitcharray of claim 1 wherein at least one of the electrodes is transparent.11. A three dimensional microswitch array comprising a stack of aplurality of the arrays of claim
 1. 12. A three dimensional microswitcharray comprising a stack of a plurality of the arrays of claim
 2. 13. Amulti-element voltage-switchable sensor array comprising a microswitcharray of claim 1 with individual elements of the array connected inseries with individual members of a plurality of sensor elements, saidsensor elements producing an electrical signal in response to a stimulusbeing sensed.
 14. The multi-element voltage-switchable sensor array ofclaim 13 wherein the sensors are thin layer sensors, themselves being infirst sensor electrode/sensor semiconductor/second sensor electrodelayer form.
 15. The multi-element voltage-switchable sensor array ofclaim 14 wherein the sensor semiconductor is an organic semiconductor.16. The multi-element voltage-switchable sensor array of claim 15wherein the microswitch and the sensor share a common electrode.
 17. Amulti-element voltage-switchable sensor array comprising a microswitcharray of claim 5 with individual elements of the array connected inseries with individual members of a plurality of sensor elements, saidsensor elements producing an electrical signal in response to a stimulusbeing sensed.
 18. The multi-element voltage-switchable sensor array ofclaim 17 wherein the sensors are thin layer sensors, themselves being infirst sensor electrode/sensor semiconductor/second sensor electrodelayer form.
 19. The multi-element voltage-switchable sensor array ofclaim 18 wherein the sensor semiconductor is an organic semiconductor.20. The multi-element voltage-switchable sensor array of claim 19wherein the microswitch and the sensor share a common electrode.
 21. Themulti-element voltage-switchable sensor array of claim 15 additionallycomprising a supporting substrate.
 22. The multi-elementvoltage-switchable sensor array of claim 15 wherein the sensor senseslight.
 23. The multi-element voltage-switchable sensor array of claim 22wherein the second sensor electrode is transparent to the light beingsensed.
 24. The multi-element voltage-switchable sensor array of claim22 wherein the light comprises visible light.
 25. The multi-elementvoltage-switchable sensor array of claim 22 wherein the light comprisesultraviolet light.
 26. The multi-element voltage-switchable sensor arrayof claim 22 wherein the light comprises infrared light.
 27. Themulti-element voltage-switchable sensor array of claim 15 wherein thesensor senses X-rays.
 28. The multi-element voltage-switchable sensorarray of claim 15 wherein the sensor senses ionized high energyparticles selected from electrons, beta particles and gamma rayradiation.
 29. The multi-element voltage-switchable sensor array ofclaim 15 wherein the sensor senses surface pressure.
 30. Themulti-element voltage-switchable sensor array of claim 15 wherein thesensor senses surface temperature.
 31. The multi-elementvoltage-switchable sensor array of claim 13 made in two dimensional x-yaddressable form.
 32. A method for driving an individual member of amulti-element voltage-switchable sensor array the sensor arraycomprising microswitch array having a plurality of individual elementsof the array connected in series with individual members of a pluralityof sensor elements and a bias voltage source, the microswitch arraycomprising a plurality of electric microswitches each member of theplurality of microswitches being in first electrode/semiconductor/secondelectrode layer form with the semiconductor layer being a unit bodyshared by the members of the array, the plurality of microswitchesdefined by a row of first electrodes and a column of second electrodesconfigured in an x-y-addressable array, and the x-y addressable positionof each individual microswitch being uniquely defined by a particularfirst electrode defining the x coordinate and a particular secondelectrode defining the y coordinate, and said sensor elements producingan electrical signal in response to a stimulus being sensed, the methodcomprising applying a positive bias voltage greater than themicroswitch's turn on voltage across the particular first electrode andparticular second electrode defining the individual member, leaving theremainder of the first electrodes and the remainder of the secondelectrodes floating and reading the electrical signal produced by theparticular sensor element.
 33. The method of claim 32 wherein multiplemembers of the array are driven serially.
 34. A method for driving anindividual member of a multi-element voltage-switchable sensor array thesensor array comprising microswitch array having a plurality ofindividual elements of the array connected in series with individualmembers of a plurality of sensor elements and a bias voltage source,said bias voltage source capable of providing a “high bias voltagestate, a 0 bias voltage state and a “low” bias voltage state, themicroswitch array comprising a plurality of electric microswitches eachmember of the plurality of microswitches being in firstelectrode/semiconductor/second electrode layer form with thesemiconductor layer being a unit body shared by the members of thearray, the plurality of microswitches defined by a row of firstelectrodes and a column of second electrodes configured in anx-y-addressable array, and the x-y addressable position of eachindividual microswitch being uniquely defined by a particular firstelectrode defining the x coordinate and a particular second electrodedefining the y coordinate, and said sensor elements producing anelectrical signal in response to a stimulus being sensed, the methodcomprising applying the positive bias to the particular first electrodeand the negative bias voltage to the particular second electrodedefining the individual member thereby exceeding said member's turn onvoltage, and applying the negative bias to the remainder of the firstelectrodes or applying the positive bias to the remainder of the secondelectrodes thereby leaving the remainder of the plurality of individualelements in an off condition and reading the electrical signal producedby the particular sensor element.
 35. The method of claim 34 whereinmultiple members of the array are driven serially.
 36. A method fordriving a series of individual member of a multi-elementvoltage-switchable sensor array the sensor array comprising microswitcharray having a plurality of individual elements of the array connectedin series with individual members of a plurality of sensor elements anda bias voltage source, said bias voltage source capable of providing afirst bias voltage and a second bias voltage the difference between thetwo bias voltages exceeding the turn on voltage for the elements of themicroswitch array, the microswitch array comprising a plurality ofelectric microswitches each member of the plurality of microswitchesbeing in first electrode/semiconductor/second electrode layer form withthe semiconductor layer being a unit body shared by the members of thearray, the plurality of microswitches defined by a row of firstelectrodes and a column of second electrodes configured in anx-y-addressable array, and the x-y addressable position of eachindividual microswitch being uniquely defined by a particular firstelectrode defining the x coordinate and a particular second electrodedefining the y coordinate, and said sensor elements producing anelectrical signal in response to a stimulus being sensed, the methodcomprising applying the first bias voltage all the first electrodes andapplying the second bias voltage bias to a particular second electrodedefining a particular column of individual elements thereby turning onthe particular column of elements and leaving the remainder of theplurality of individual elements in an off condition and reading theelectrical signal produced by the particular column of sensor elements.37. The method of claim 33 wherein the reading of the electrical signalproduced by the particular column of sensor elements is carried out witha digital shift register or a with a digital decoder to produce a seriesof electrical signals corresponding to the electrical signals generatedby the series of sensor elements in the particular column.
 38. Aplurality of sensor arrays of claim 13 stacked to form a threedimensional matrix.
 39. A plurality of sensor arrays of claim 13 stackedto form an integrated sensor array with multiple sensing functions. 40.The method for producing an array of claim 6 comprising laying down anelectrode-semiconductor-electrode electric microswitch on top of anarray of sensor elements.
 41. The method for producing an array of claim6 comprising laying down an array of sensor elements on top of an arrayof electrode-semiconductor-electrode electric microswitches.